The present invention relates generally to liquid crystal display (LCD) devices, and more particularly to a system, apparatus and method for improving image quality by limiting the difference between gray scale values of adjacent pixels.
Liquid crystal displays (LCDs) are commonly used in devices such as portable televisions, portable computers, control displays, and cellular phones to display information to a user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission. When used as a high resolution information display, as in one application of the present invention, LCDs are typically arranged in a matrix configuration with independently controlled display areas called xe2x80x9cpixelsxe2x80x9d (the smallest segment of the display). Each individual pixel is adapted to selectively transmit or block light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode).
A LCD pixel can control the transference for different wavelengths of light. For example, an LCD can have pixels that control the amount of transmission of red, green, and blue light independently. In some LCDs, voltages are applied to different portions of a pixel to control light passing through several portions of dyed glass. In other LCDs, different colors are projected onto the area of the pixel sequentially in time. If the voltage is also changed sequentially in time, different intensities of different colors of light result. By quickly changing the wavelength of light to which the pixel is exposed an observer will see the combination of colors rather than sequential discrete colors. Several monochrome LCDs can also result in a color display. For example, a monochrome red LCD can project its image onto a screen. If a monochrome green and monochrome blue LCD are projected in alignment with the red, the combination will be a full range of colors.
The monochrome resolution of an LCD can be defined by the number of different levels of light transmission or reflection that each pixel can perform in response to a control signal. A second level is different from a first level when a user can tell the visual difference between the two. An LCD with greater monochrome resolution will look clearer to the user.
LCDs are actuated pixel-by-pixel, either one at a time or a plurality simultaneously. A voltage is applied to each pixel area by charging a capacitor formed in the pixel area. The liquid crystal responds to the charged voltage of the pixel capacitance by twisting and thereby transmitting a corresponding amount of light. In some LCDs an increase in the actuation voltage decreases transmission, while in others it increases transmission. When multiple colors are involved for each pixel, multiple voltages are applied to the pixel at different positions (different capacitance areas being charged of a pixel) or times depending upon the LCD illumination method. Each voltage controls the transmission of a particular color. For example, one pixel can be actuated for only blue light to be transmitted while another for green light, and a third for red light. A greater number of different light levels available for each color results in a much greater number of possible color combinations. Colors may be combined from a red pixel, a green pixel and a blue pixel, each residing on a different LCD, to produce any desired combined pixel color. The three LCDs (red-green-blue or RGB) are optically aligned so that the resulting light from each of the corresponding RGB pixels produces one sharp color pixel for each of the pixels in the LCD pixel matrix. The LCD pixel matrix is adapted for displaying one frame of video per light strobe. Each light strobe (RGB) produces one video frame. A sequence of video frames produces video images that may change over time (e.g., motion video).
Converting a complex digital signal that represents an image or video into voltages to be applied to charge the capacitance of each pixel of an LCD involves circuitry that can limit the monochrome resolution. The signals necessary to drive a single color of an LCD are both digital and analog. It is digital in that each pixel requires a separate selection signal, but it is analog in that an actual voltage is applied to charge the capacitance of the pixel in order to determine light transmission thereof.
Each pixel in the array of the LCD is addressed by both a column (vertical) driver and a row (horizontal) driver. The column driver turns on an analog switch that connects an analog voltage representative of the video input (control voltage necessary for the desired liquid crystal twist) to the column, and the row driver turns on a second analog switch that connects the column to the desired pixel.
The video inputs to the LCD are analog signals centered around a center reference voltage of typically from about 6.5 to 8.0 volts. A voltage equal or close to this center reference voltage is called xe2x80x9cVCOMxe2x80x9d and is supplied to the LCD Cover glass electrode which is a transparent conductive coating on the inside face (liquid crystal side) of the cover glass. This transparent conductive coating is typically Indium Tin Oxide (ITO).
One frame of video pixels are run at voltages above the center reference voltage (positive inversion) and for the next frame the video pixels are run at voltages below the center reference voltage (negative inversion). Alternating between positive and negative inversions results in substantially a zero net DC bias at each pixel. This substantially reduces the xe2x80x9cimage stickingxe2x80x9d phenomena.
LCD technology has reduced the size of displays from full screen sizes to minidisplays less than 1.3 inches diagonal measurement, to microdisplays that require a magnification system. Microdisplays may be manufactured using semiconductor integrated circuit (IC) dynamic random access memory (DRAM) process technologies. The microdisplays consist of a silicon substrate backplane, a cover glass and an intervening liquid crystal layer. The microdisplays are arranged as a matrix of pixels arranged in a plurality of rows and columns, wherein an intersection of a row and a column defines a position of a pixel in the matrix. To incident light, each pixel is a liquid crystal cell above a reflecting mirror. By changing the liquid crystal state, the incident light can be made to change its polarization. The silicon backplane is an array of pixels, typically 10 to 20 microns in pitch. Each pixel has a mirrored surface that occupies most of the pixel area. The mirrored surface is also an electrical conductor that forms a pixel capacitor with the ITO layer as the other plate of the pixel capacitor (common to all pixel capacitors in the matrix of pixels. As each pixel capacitor is charged to a certain pixel value, the liquid crystals between the plates of the pixel capacitors xe2x80x9ctwistxe2x80x9d or xe2x80x9cuntwistxe2x80x9d which affects the polarization of the light incident to the pixels (reflections from the pixel mirrors).
Microdisplays may have an analog video signal input (xe2x80x9canalog displayxe2x80x9d) or a digital video signal input (digital display). Analog displays, generally, are addressed in a raster mode, while the pixels in a digital display may be addressed like a DRAM, in a random order. Random access allows updating only pixels requiring updating, thus saving on processing time and associated power consumption.
A problem exists in small LCDs, especially microdisplays, which have small pixel cell areas compared to the area of the gaps between the pixel cells. Fringe fields between the pixels are therefore significant in magnitude and the area affected by fringe fields is significant with respect to the overall pixel area. This leads to image degradation of increasing severity for small LCDs and high driving voltages. Limiting the driving voltages helps, but reduces the available contrast of the LCD.
The present invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing a system, method and apparatus for improving image quality of a liquid crystal display (LCD) by modifying the video source values written to the pixels in order to smooth the magnitude of voltage transitions from one adjacent pixel to another. If the voltage transitions between adjacent pixels is too large in magnitude, the large voltage transition can generate a strong fringe field effect between the adjacent pixels.
A liquid crystal on silicon (LCoS) microdisplay is adapted to receive video information from a digital video data source. The LCoS microdisplay may operate, e.g., in a normally white twisted nematic LC mode. A rubbing direction may be selected so that disclinations appear preferably at vertical pixel borders (between columns), e.g., a 60 degree twist self-compensated reflective twisted nematic mode. If a source image with black areas surrounded by light gray areas is displayed, a white line may be observed within a gray area that borders the black area on one side thereof, while on the other side of the black area a white spot may be observed therein. If the source video image for the pixels at the border of the gray/black areas are modified, e.g., a normally black pixel written more toward gray (lighter than black but darker than the normal gray), or a gray pixel written more toward black (darker), then the resulting LCD video image has significantly less image distortion due to fringe effect fields. Such a slight reduction in the blackness of a pixel or reduction of lightness of a pixel next to a black pixel has a strong effect in the applied voltage since the electro-optical response of the liquid crystal has a small gradient close to the saturation voltage for a black pixel.
For exemplary purposes in describing the embodiments disclosed herein, a pixel voltage value (the voltage value charge on the pixel capacitor) representing black may be referred to as black or level A (00h input to an 8-bit DAC), and a pixel voltage value representing white may be referred to as white or level D (FFh input to the 8 bit DAC). Gray levels may be referred to as gray or level C (greater than blackxe2x80x9400h and less than whitexe2x80x94FFh to the 8 bit DAC). 00h is 0 in base 10 and FFh is 255 in base 10.
There is a liquid crystal (LC) defect called xe2x80x9cdisclinationxe2x80x9d on a border where deep black pixels meet brighter (lighter or whiter) pixels. Top and bottom borders generally are not affected, but left and right borders may emit a bright line. A grayshade of 60 (out of 255) at a border of grayshade 10 (out of 255) is hardly noticeable, however, a grayshade (gs) of 60 at a border of grayshade 0 is very noticeable. A solution, according to the present invention, is to display black as gs 0 when it detects a fairly large swath of black. For example, given a gray flatfield of gs 60 with a broad black (grayshade 0) line running vertically across it. If the black line were 20 columns wide, the first few columns may be written, for example but not limited to, gs 15, gs 11, gs 7, gs 4, gs 2, gs 1 and finally gs 0. When coming out of the 20 column wide swath, the last few columns may be written, for example but not limited to, gs 1, gs 2, gs 4, gs 7, gs 11 and gs 15. Simply halving, e.g., dividing by two, the pixel gs values accomplishes the intended purpose of the present invention. Other divide ratios may be effectively used and are contemplated here.
An additional feature of the invention controls at what point the adjacent pixel gs values are xe2x80x9csoftenedxe2x80x9d or rounded. A limit may be defined which is used to restrict the range of gs values being divided by two. For example, if the limit were set to zero, all gs values would pass through the video stream without change. Conversely, if the limit where set to 255, then the divide-by-two operation would occur under all circumstances, e.g., whenever adjacent pixel gs values vary by more than a factor of two. For example an adjacent pixel pair transformation may be represented as follows:
An intermediate limit, e.g., 64, would not affect pixel gs values greater than 64. For example:
In an exemplary embodiment of the invention, eight registers may be used in performing the xe2x80x9cdivide-by-twoxe2x80x9d operations on the pixel gs values. It is contemplated and within the scope of the present invention that more or less than eight registers may be used to perform the divide-by-two operations. One implementation of the xe2x80x9cdivide-by-2xe2x80x9d algorithm may be as follows. Assign the input video pixel stream to 8 registers, A through G:
Gxe2x86x92Exe2x86x92Cxe2x86x92A :Aprmxe2x86x92Aprmdly
Hxe2x86x92Fxe2x86x92Dxe2x86x92B :Bprmxe2x86x92Bprmdly
Where {Bprm, Aprm} are the transformed pixel values at the same time point as {B, A} The arrows indicate where the clock steps are. {Bprmdly, Aprmdly} is just the solution of the previous pixel input which is needed to help in the calculation of Aprm and Bprm. Aprm and Bprm can be solved mathematically by the following:
Amin=MAX{min(H/128), limit/64), min (G/64, limit/32), min(F/32, limit/16), min(E/16, limit/8), min(D/8, limit/4), min(C/4, limit/2), min(B/2, limit), min(Bprmdly/2, limit)}
then if A less than Amin, Aprm=Amin; else Aprm=A
Bmin=MAX{min(H/64), limit/32), min (G/32, limit/16), min(F/16, limit/8), min(E/8, limit/4), min(D/4, limit/2), min(C/2, limit), min(Aprm/2, limit)}
then if B less than Bmin, Bprm=Bmin; else Bprm=B
The above equations may be implemented with digital logic, e.g., FPGA, PLA, ASIC, microcontroller, microprocessor and the like using reduced terms as follows:
term0=maximum(trunc(H/4), trunc(G/2))
term1=minimum(trunc(term0/16), trunc(limit/16))
term2=minimum(trunc(term0dly/4), trunc(limit/4)) (where term0dly is delayed one clock)
term3=minimum(term0dlydly, limit)(where term0dlydly is delayed two clocks)
term4=minimum(trunc(Aprm/2), limit)
term5=minimum(trunc(B/2), limit)
term6=minimum(trunc(Bprmdly/2), limit)
Amin=maximum(trunc(term1/2), trunc(term 2/2), trunc(term 3/2), term 5, term6)(calculate this before Bprm)
Bmin =maximum(term1, term2, term3, term4)
If A less than Amin, then Aprm=Amin; else Aprm=A
If B less than Bmin, then Bprm=Bmin; else Bprm=B
In an alternate embodiment, the above equations may be modified slightly to remove the Bmin dependency on Aprm and use terms based on A and Bprm_dly in its stead as follows:
Bmin=MAX{min(H/64), limit/32), min (G/32, limit/16), min(F/16, limit/8), min(E/8, limit/4), min(D/4, limit/2), min(C/2, limit), min(A/2, limit), min(Bprmdly/4, limit/2)}
Table 1 hereinbelow depicts some test vectors to illustrate how an exemplary embodiment of the invention may function. The vectors are depicted in single file, e.g., H, G, F, E, D, C, B, A rather than two-pixels per clock: e.g., {H,G}, {F,E}, {D,C}, {B,A}.
For a sequential color LCD system, only one set of shift registers need be used. For a three color (red-green-blue) LCD system, three sets of shift registers may be used, one for each color portion of the RGB LCDs.
Adjacent pixels on the same row may be as described herein as well as adjacent pixels on adjacent rows. It is contemplated and within the scope of the present invention that a video memory may be utilized to store voltage values written to pixels on previous rows and/or columns so that no adjacent pixel has a voltage value difference great enough to cause field fringe effects.
The present invention is directed to a system for improving image quality of a liquid crystal display (LCD), said system comprising: a matrix of pixels arranged in a plurality of columns and a plurality of rows, wherein an intersection of a row and a column defines a location of a pixel in said matrix; at least one digital-to-analog converter (DAC) having a digital input and an analog output; a plurality of column switches adapted for coupling the analog output of said at least one DAC to each of said plurality of columns; a plurality of row switches adapted for selectively coupling each of said plurality of rows to said plurality of columns; column control logic for controlling said plurality of column switches; row control logic for controlling said plurality of row switches; a video frame to gray scale conversion and pixel address logic for converting video information into LCD gray scale values and corresponding pixel address locations thereof; and video data comparator/modifier logic, said video data comparator/modifier logic adapted to receive the LCD gray scale values for each pixel of the matrix of pixels, wherein gray scale values of adjacent pixels are compared and if a ratio of the gray scale values of adjacent pixels is greater than a desired value, then one of the gray scale values is modified so that the ratio of adjacent pixel gray scale values is no greater than the desired value; said video data comparator/modifier logic is adapted for sending all unmodified gray scale values and any modified gray scale values to said at least one DAC; said video frame to gray scale conversion and pixel address logic adapted for sending said pixel address locations to said column control logic and said row control logic.
The present invention is also directed to a method for improving image quality of a liquid crystal display (LCD) comprising a matrix of pixels arranged in a plurality of rows and columns, wherein an intersection of a row and a column defines a position of a pixel in the matrix, said method comprising the steps of: determining if a ratio of gray scale values of adjacent pixels is greater than a desired value, wherein; if the ratio is less than or equal to the desired value, then writing the gray scale values to the adjacent pixels, and if the ratio is greater than the desired value, then modifying one of the gray scale values of the adjacent pixels so that the ratio is less than or equal to the desired value, and then writing a gray scale value and the modified gray scale value to the adjacent pixels.
A technical advantage of the present invention is improved image quality in microdisplays. Another technical advantage is in smoothing transitions between pixel voltages that generate strong fringe field effects. Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Various embodiments of the invention obtain only a subset of the advantages set forth. No one advantage is critical to the invention.